1. Field of the Invention
The present invention relates to a welding method for welded joints in high strength, heat resistant steels, especially in high strength austenite stainless steels and high strength ferrite type heat resistant steels. In particular, it relates to a multi-pass buildup weld joint and welding method for high strength, heat resistant steels as well as a multi-pass buildup weld joint and welding method for high strength ferrite type heat resistant steels which improve the high temperature strength of the welded joints.
2. Description of the Related Art
Recently, improved electricity generation efficiency in thermal power plants has raised the temperatures and pressures for steam generation, which, in turn, has increased the use of high strength, heat resistant steels. Among such high strength, heat resistant steels, high strength austenite steels, and from the perspectives of economics and thermal expansion coefficients, the recently developed high strength ferrite type heat resistant steels based on tempered martensite compositions, have been used.
In particular, the high strength ferrite type heat resistant steels have low thermal expansion coefficients not seen in austenite steels, and have additional endurance, including high pressure resistance, resistance to stress and corrosion fractures, and resistance to peeling of their oxide scale. These advantages are not seen in austenite steels, and their low alloy content also makes them economical.
A great number of high chromium ferrite steels have been developed for use in heat resistant materials such as boilers and in heat conducting steel piping. These steels took advantage of the inherent strengths of conventional ferrite type heat resistant steels while offering further improvements in high temperature strength, corrosion resistance, and resistance to steam corrosion.
For example, Japan Patent Publication Hei 3-97832 and Japan Patent Publication Hei 5-311345 disclose technology of exceptional steels which retain adequate strength, corrosion resistance, and resistance to steam corrosion even in high temperature environments of 600 and above.
However, the steels disclosed above and the conventional high chromium ferrite steels were still plagued by a single unresolved problem, the “high temperature strength of welded joints.”
Although Japan Patent Publication Hei 9-13150 disclosed a high chromium ferrite steel with excellent creep properties in its welded joints, for other generally used steels, the problem of high temperature strength in weld joints, caused by the welding heat inducing structural changes in the heat affected area of the weld joint, remained unresolved.
The aforementioned heat affected area is that area of the weld where heat influx during welding causes structural changes, and those structural changes are determined by the maximum heating temperature generated by the welding heat source and the rate of cooling.
To wit, the base metal area is affected by the welding heat from the welding line at the boundary between the base metal and the weld filler metal from successive temperature changes from high temperature just below its melting point to low temperatures. After rapid heating, the subsequent rapid cooling causes transformation, deposition, recovery, recrystallization, crystal growth, annealing, tempering or other metallurgical changes.
Further, since the welding heat source causes heating to high temperatures and then rapid cooling, the structure of the base metal in the weld heat affected area differs from the original structure of the base metal. In the case of high strength ferrite steels, this is one of the causes of a decline in high temperature strength properties in the weld joint, including the foregoing heat affected area.
FIG. 6(A) shows a welded joint and the groove for a weld made on the aforementioned high strength, heat resistant steel base metal using conventional welding techniques; FIG. 6(B) shows the creep rupture status.
As shown in FIG. 6(A), weld joint 55 was formed in groove 51 having root 52 using multiple pass layers.
Brittle fracture area 54 for the weld joint having the above described structure, as shown in FIG. 6(B), is within heat affected area 53.
The strength characteristics between base metal 10 and weld joint 55 for the foregoing conventional case are shown in FIG. 7. As is shown in the figure, the properties change according to time and temperature. In the prior art, reductions in creep fracture stress of 10 to 15% with respect to the stress of the base metal were seen.
FIG. 8 is a structural diagram showing a welding method, disclosed in Japan Patent Publication Hei 7-9147, for preventing brittleness in the heat affected area of highly pure ferrite stainless steels. In this proposal, a multi-pass buildup is performed by covered arc welding. The weld filler metal is built up to the final layer, and then the base metal surface in the heat affected area is fused using a pass from a TIG arc. The TIG arc, not requiring any flux to remove oxides in the aforementioned heat affected area, prevents brittleness in the heat affected area.
As described above, a structural change inevitably takes place in the aforementioned heat affected zone in the base metal when the weld filler metal is used to make a welded joint which causes, compared with the base metal, the high temperature strength of the welded joint overall, as described using the foregoing figures, to undergo about 10 to 15% stress reduction. Although a great deal of research has gone into the mechanism for this reduction and for improving the base metal component, the problem remains unresolved.
Accordingly, when high strength heat resistant steels, including the aforementioned high strength ferrite heat resistant steels, are used to fabricate high temperature equipment, the overall design for the piping and steel plate must be about 10% thicker to compensate for the decline in the high temperature strength of the welded joints.
Accordingly, enormous losses derive from the amount of material that needs to be used, and economy is lost. Also, incidents of fracture in the foregoing heat affected area of welded joints have been reported, and since rupture of such high temperature, high pressure equipment can cause life-threatening accidents, the problem is in urgent need of resolution.